I'm no expert, but here goes. I've read your posts here, and am impressed with your knowledge - so I hope what I have to say is helpful and not too elementary.

Classical REBT assumes the worst. "If you were to go to a social event and urinate your pants, what would that mean to you?" The key is to convince her that even if the worst possible thing happens, it is not awful or catastrophic. Embarassing, yes - but not horrible. Help her develop skills for coping with urinating in her pants - rather than convince her she's not going to do it. Discuss the use of humor to help her out of the embarassing situation: "This party's so great I can't control myself! Pardon me while I change."

One thing you can do to help with anti-awfulizing is what REBT therapists call the Catastrophe Scale: have her list the most horrible, catastrophic things she can think of, such as tornadoes, hurricanes, extreme disfigurement, mass bombings, etc. Oh, yes - I forgot one - urinating your pants in public. This helps put the A in perspective, and helps facilitate more rational beliefs about A (such as, "it would be embarassing and humiliating, but not the end of the world," or "while it's humiliating, I certainly can survive such a happening.") Another would be to have her imagine yourself or one of her friends having such an accident in public, and ask her how she would feel about witnessing that. She probably would feel compassion and empathy rather than judgement. Then you can ask her why the rules are different for you or her friends than for herself.

I'm guessing Ellis would have her do Rational Emotive Imagery, and he would probably say something like, "Let's imagine you go to a party and you do urinate on yourself, which you're probably not going to do anyway, but let's just imagine that you do....." (Imagine that nasal New York twang of his)

I'll think about this case for a while and get back to you if I come up with any more ideas.